The invention belongs to the field of a chemical industry, and particularly relates to a preparation method of a zinc oxide nanocrystal, so that the photocatalytic activity of zinc oxide is improved.The method comprises the steps of firstly preparing TEMPO oxidized cellulose, then respectively grafting a polymer containing carboxyl, a polymer containing sulfonyl, or a polymer containing an epoxygroup through a heterogeneous free radical polymerization method, preparing to obtain modified TEMPO oxidized cellulose, then adopting the modified TEMPO oxidized cellulose as a template, adsorbing aprecursor zinc acetate of zinc oxide, obtaining cellulose/ zinc oxide compound under hydrothermal reaction, calcining to remove the template, and finally obtaining the zinc oxide nanocrystal. The ZnOnanocrystal prepared through the method provided by the invention can replace an existing ZnO photocatalytic material, has the characteristics of high wurtzite crystal form content, large specific surface area and large pore volume, shows a better photocatalytic performance, is simple in preparation process, and belongs to an environment-friendly material.